Slotted wave guide aerials



1 70 R. A. WALDRON 3,530,479

SLOTTED WAVE GUIDE AERIALS Original Filed March 31, 1967 I5Sheets-Sheetl INVENTOR W W Wm .87 MATTORNEYS Sept. 22, 1970 R. A.WALDRON 3,530,479

SLOTTED WAVE GUIDE AERIALS Original Filed March 31, 1967 3 Sheets-Sheet2 INVENTOR W m MM 3r MZM M ATTORNEYS P 1970 R. A. WALDRON 3,530,479

SLOTTED WAVE GUIDE AERIALS Original Filed March 31, 1967 3 Sheets-Sheet5 INYENTOR M flm/Jaldm ,51 4/ ATTORNEYS United States Patent US. Cl.343771 Claims ABSTRACT OF THE DISCLOSURE The disclosure herein relatesto slotted waveguide aerials, i.e. a waveguide having one or more slotsin it through which radiation takes place, and discusses known aerialsof this type and points out that their bandwidth is undesirably lowabout12% at the most. Aerial arrays of the directional coupler type aredisclosed to give a bandwidth of about 15% and bandwidths substantiallylarger than this are obtainable from leaky waveguide arraysthe latterare diflicult and costly to construct and have a low efliciency. Theinvention disclosed herein provides aerials and aerial arrays which aresimple and efiicient and have a bandwidth of about The invention employsa waveguide having a parallel-sided slot in one wall, the distance ofthe slot from the edges of the wall varying at least approximatelysinusoidally along the length of the slot. The sinusoidal variation maybe achieved by making the slot straight and the edges of the wallsinusoidal.

Aerial arrays incorporating slotted waveguide aerials in accordance withthe invention are also disclosed.

This application is a division of application Ser. No. 627,554, filedMar. 31, 1967, now abandoned, originally in the names of MatthewFrederick Radford and Richard Arthur Waldron, for Slotted Wave GuideAerials.

This invention relates to slotted waveguide aerials, i.e. to aerials ofthe kind constituted by a waveguide having one or more slots cut in awall thereof, radiation taking place through the slot or slots when thewaveguide is fed with microwave energy. The object of the invention isto provide improved slotted waveguide aerials which, though simple andrelatively easy to manufacture, shall be relatively wide band.

Slotted waveguide aerials are, of course, well known, but such aerialsas at present known and used in practice are undesirably limited as totheir bandwidths. In general they have bandwidths of between 6% and 12%(depending upon their designs and type), and, so far as the presentapplicant is aware, a bandwidth of 12% is about the largest figureattainable with known slotted waveguide aerials as now in use. Knownaerial arrays of the directional coupler type can be designed to giveslightly better bandwidthsup to about 15%but the bandwidth improvementover slotted waveguide aerials is not very great and they are expensiveand difficult to construct. When substantially larger bandwidths arerequired, it is usual, at the present time, to resort to what arecommonly called leaky waveguide arraysi.e. waveguides in which one ofthe two narrower walls is replaced by a grid of wires runningperpendicularly to the length of the guide. This type of aerial is quitecostly and diflicult to construct and has the serious defect of being ofloW efficiency because the beam emerges obliquely. The present inventionprovides aerials of relatively simple and satisfactory constructionmechanically which are of good efliciency and capable of being designedto have bandwidths in the neighborhood of 30%a very substantially3,539,479 Patented Sept. 22, 1970 larger bandwidth than is attainablewith known slotted waveguide aerials.

According to the invention disclosed in said application Ser. No.627,554, a wide band slotted waveguide aerial comprises a waveguidewhich is of such cross section that it will propagate microwave energyin only the principal mode at which the guide cut-01f frequency islowest and which has at least one fiat or approximately flat wall and,in said wall, a substantially parallel sided slot the distance of whichforms an adjacent longitudinal extremity of said wall varies at leastapproximately sinusoidally along the length of said slot.

The expression approximately sinusoidal and similar expressions areherein employed to include not only sinusoidal shapes but also shapesconsisting of a succession of straight line portions which togetherapproximate to the sinusoidal. In the latter case the corners where thestraight line portions meet may be smoothed or rounded off.

The waveguide may be of any of a variety of different cross sectionalshapes, e.g. the said shape may be simple rectangular or it mayapproximate to a flattened ellipse or a so-called ridged waveguide maybe used, the slot being, in all cases, in one broad wall.

According to the present invention, the approximately sinusoidalvariation of distance of the slot in a slotted wall of the guide from anadjacent longitudinal extremity of said wall can be obtained by makingsaid extremity approximately sinusoidal and said slot straight.

The slot width is not critical. A practical range for slot width is fromg/SO to g/ 10 where g is the length of the wave in the guide. Very goodresults are obtainable with a slot width of g/ZO.

Slotted waveguide aerials in accordance with this invention are intendedto be end fed, i.e. a radio transmitting and/or receiving system inaccordance with the invention would comprise a slotted waveguide aerialaccording to the said invention and radio transmitting and/ or receivingapparatus coupled to one end of the guide. i

All parts of the approximately sinusoidal slot of an aerial inaccordance with this invention contribute to the radiation, the portionswhich are or can be regarded as approximately inclined to the axis ofthe guide doing so because they are at a small angle to the longitudinalcurrents in the guide and the other portions, which are or can beregarded as mutually displaced portions approximately parallel to theaxis of the guide doing so because by virtue of the displacement theycouple with the lateral currents in the guide.

The amount of power radiated depends upon the amplitude of theapproximate sinusoid and can be controlled in design between widelyspaced limits by suitably choosing this amplitude. I

If the period of the sinusoid is equal to the wavelength in the guidethe direction of maximum radiation is at right angles to the guide axis,i.e. the aerial is a broadside one. If the wavelength in the guide ischanged from this value the direction of maximum radiation will be swungso as to incline to the broadside direction. Accordingly, by suitablychanging the frequency in the guide, the direction of maximum radiationmay be varied over a usefully wide angle extending on both sides of thebroadside direction.

Aerials in accordance with this invention may be stacked to form aerialarrays either with their broad walls (containing the slots) coplanar orspaced apart and parallel to one another and perpendicular to a certainplane. In the latter case, sideways extending feeder guides are used toconvey the microwave energy from the slots in a direction parallel tothe broad walls of the waveguides and perpendicular to the principalaxes of the guides so as to deliver the energy to horns with theirsmaller ends situated in said certain plane. 'If this is done, however,the path lengths from the slots to the smaller ends will be variablewith a spatial periodicity equal to the spatial periodicity of theslots, thus causing the same spatial periodicity in the phase of theradiated waves which may produce an undesired radiation pattern. A moredesirable radiation pattern can be obtained by interposing cavitiesbetween the waveguides and the feeder guides and so arranging mattersthat the path length from the waveguide slot to the cavity slot andthence to the smaller end of the horn has a periodicity twice that ofthe waveguide slots. This results in a more desirable radiation patternover bandwidths up to about 30%. If, however, such an arrangement isused with a bandwidth exceeding about 30%, undesirable effects areliable to occur due to coupling into a new undesired mode of propagationarising in the structure. This difficulty can be overcome by using awaveguide which is sinusoidal or approximately so and has a straightslot in it as hereinbefore described. The need for the cavity is theneliminated since the path length from the slot to the smaller end of thehorn is constant.

The invention is illustrated in the accompanying drawings which showseveral embodiments of the invention by way of example. In thesedrawings, FIGS. 1, 2, and 3 show waveguide aerials as disclosed in theparent application Ser. -No. 627,554 and which may be used in accordancewith the present invention, FIG. 4 shows an array of such waveguideaerials, and FIG. 5 and 6 show waveguide aerials according to thepresent invention.

Referring to FIG. 1, this shows an embodiment consisting of a simplerectangularly sectioned guide G of standard dimensions having asinusoidal slot S cut in one broad wall B thereof. The dimension L isthe period of the sinusoid. 'If this is equal to Ag (the guidedwavelength) the aerial will radiate broadside. A good value for thewidth of the slots S is Ag/ZO.

FIG. 2 shows a modification differing from that of FIG. 1 only in thatthe slot, here referenced SS, is not a true sinusoid but onlyapproximately sinusoidal. It consists of a succession of straight lineportions which together approximate to the sinusoidal. In the particularembodiment shown in FIG. 2, the portions comprise portions such as Iwhich are inclined to the guide axis and portions such as D which areparallel to the guide axis and displaced with respect to one another.

FIG. 3 shows a modification differing from that of FIG. 2 only in thatthe rectangular guide G, instead of being of simple rectangular crosssection, is ridged as known per se, having a ridge R so that it is ofreduced height where the ridge occurs. Obviously, the guide of FIG. 1could be ridged like the guide of FIG. 3.

FIG. 4 illustrates an aerial array comprising a number of slottedwaveguide aerials, each having a cavity with a straight slotted cavitywall associated therewith, each aerial having a sinusoidal orapproximately sinusoidal slot in a broad wall thereof. These aerialscould be as illustrated in any of FIGS. l3. In FIG. 4, the guides areshown in end view and referenced G1, G2 and G3. There may be any numberof such guides in the array, but it is sufficient, for the purpose ofillustration, to show only three. .Each guide has a sinusoidal orapproximately sinusoidal slot S1, S2 or S3 which is covered by and opensinto a cavity C1, C2 or C3 having a straight slot SCI, 8C2 or 8C3 in thewall thereof remote from the guide. These slots couple into sidewaysextending feeder guides F 1, F2 or F3 which may terminate in horns orflares H1, H2 or H3 as shown. Such an array has the property that thepath length from S1, S2 or S3 via SCI, SC2 or 8C3 respectively to theend of F1, F2 or F3 respectively adjacent to H1, H2 or 1-13 respectivelyin the same whether S1, S2 or S3 are in the positions shown in FIG. 4 orare an equal distance from the centers of the broad walls in which theyare situated 'but are on the opposite sides of the centers. The resultis that the periodicity of the path lengths and thus of the phases atthe ends of F1, F2, F3 is 4 twice that of the slots S1, S2, S3. Thisarrangement has the defect, above mentioned, that an unwanted mode canexist and be excited if operation over a bandwidth exceeding about 30%is required.

FIG. 5 shows in perspective a modification of an aerial as shown inFIG. 1. It consists of a rectangularly sectioned guide WG having a slot86 in one broad wall thereof, but, instead of the guide being straightand the slot sinusoidal, the slot is straight and the guide sinusoidal.

FIG. 6 shows a modification generally similar to that of FIG. 5, butdiffering therefrom only in that the guide section is not rectangular,but like a flattened ellipse, the broad walls of the guide beingcontinued into and joined by outwardly curved narrow walls of arcuate orapproximately arcuate shape. The sections of the guides shown in any ofFIGS. 1-3 may similarly depart from the rectangular, e.g. they maysimilarly have outwardly curved narrow walls.

According to the present invention, aerials as shown in FIG. 2 or 3 mayalso be modified by substitution of a sinusoidal or approximatelysinusoidal guide with a straight slot for a straight guide with asinusoidal or approximately sinusoidal slot. In other words, the guideof FIG. 5 or 6, or a ridged 6, can be modified to have broad wall edgeswhich are not sinusoidal but are composed of a series of straight linesproducing an approximation to a sinusoid. The corners where suchstraight lines adjoin may be rounded or smoothed off to avoid undesiredreflections from what would otherwise be sharp corners. The illustratedarrangements with sinusoidal guides are, however, at present preferred.They are not unduly difficult to manufacture by machining from thesolid. Aerials as exemplified by FIGS. 5 and 6 can be stacked to form anarray. Because the distance from the slot to the smaller end of the hornis constant, there is no need for the cavity illustrated in FIG. 4 anddifliculties caused by coupling into an unwanted mode are avoided. Itwill be seen that, when this is done, the defiect of an array as shownin FIG. 4namely undesired non-uniform phase distribution etfects-isavoided since, the slot in each guide being straight and parallel tothat in the associated cavity wall, there are no path length diiferencesto cause such effects.

Although the invention may be carried into effect using waveguides ofthe at present standard rectangular cross sections, it is, as willalready have been seen, not limited to the employment of such standardsized rectangular guides and other sectional shapes and sizes may beused. For example, very good results have been experimentally achievedusing rectangular guides of reduced height (as compared to standardguides) the narrow dimension of the rectangular section being only aboutone-quarter of the broad dimension instead of the more usual one-half,

I claim:

1. A wide band slotted waveguide aerial comprising a waveguide which isof such cross section that it will propagate microwave energy in onlythe principal mode at which the guide cut-off frequency is lowest andwhich has at least one substantially fiat wall and, in said wall, asubstantially parallel sided slot the distnce of which from longitudinalextremities of said wall varies at least approximately sinusoidallyalong the length of said slot, and in which said longitudinalextremities of said wall are substantially sinusoidal and said slot issubstantially straight.

2. An aerial as claimed in claim 1, wherein the width of the slot isapproximately one-twentieth of the guided wavelength.

3. A wide band slotted waveguide aerial as defined in claim 1 whereinthe width of the slot is within a range from about one-fiftieth to aboutone-tenth of the guided wavelength.

4. A wide band slotted waveguide aerial as defined in claim 1 whereinthe periods of the substantially sinusodial longitudinal extremities aresubstantially equal the period of the guided wavelength.

5. A wide band slotted waveguide aerial as defined in claim 3 whereinthe periods of the substantially sinusoidal longitudinal extremities aresubstantially equal to the period of the guided Wavelength.

6. A wide band slotted waveguide aerial as defined in claim 2 whereinthe periods of the substantially sinusoidal longitudinal extremities aresubstantially equal to the period of the guided wavelength.

7. An aerial array comprising a plurality of wide band slotted waveguideaerials, each of which comprises a waveguide which is of such crosssection that it will propagate microwave energy in only the principalmode at which the guide cut-off frequency is lowest and which has atleast one fiat or approximately fiat wall and, in said wall, asubstantially parallel sided slot the distance of which from an adjacentlongitudinal extremity of said wall varies at least approximatelysinusoidally along the length of said slot, said aerials being stackedside-by-side and spaced apart, each guide having a slotted cavitystructure fitted over the slot in said guide, the slot in the cavitystructure providing coupling with a sideways extending guide, each pairof slotted guides having, in the space between them, the cavitystructure and the sideways extending guide associated with one slottedguide of the pair, all said sideways extending guides being parallelwith one another so that a beam-like polar radiation diagram isobtained.

References Cited UNITED STATES PATENTS 2,676,257 4/1954 Hebenstreit343771 X 2,681,990 6/1954 Purcell 343-771 3,039,097 6/1962 Strumwasseret al. 343-771 FOREIGN PATENTS 580,377 9/1946 Great Britain. 983,0332/1951 France. 1,006,104 1/1952 France.

HERMAN K. SAALBACH, Primary Examiner S. CHATMON, JR., Assistant ExaminerUS. Cl. X.R. 343-767

